This invention relates to a transformer, particularly for a voltage converter, comprising a primary winding having a predeterminable leakage inductance and at least one secondary winding magnetically coupled to the primary winding in the predetermined voltage-transformation ratio.
In a transformer with a primary and a secondary winding and a core preferably formed from a magnetically conducting material, the value of the leakage inductances is determined by the number of turns of the individual windings and by the spatial arrangements of these windings. The leakage inductance increases with an increasing number of turns and with an increasing distance between the windings. The voltage transformation ratio, the magnetizing inductance and the losses occurring in the transformer, as well as the resultant increase of temperature, determine the number of turns for the primary and secondary winding in the dimensioning of the transformer. Due to these influences, limits are imposed on the dimensioning of a transformer, particularly as regards the maximum admissible number of turns. Moreover, the possibilities of varying the spatial arrangement of the windings are limited due to the core chosen for the relevant transformer. It has been found that the achievable values for the leakage inductances are thereby also limited. Particularly if such a transformer is used as a resonant element in a resonant-circuit power supply, it may occur that the value of the leakage inductance achievable with such a transformer cannot be dimensioned high enough. To achieve a sufficiently high leakage inductance, it will then be necessary to provide an additional coil or to choose a core for the transformer which is larger than would have to be dimensioned in accordance with the requirements for normal power transformation.
A transformer, particularly for a resonant power supply, is known from FR 2 730 342-A1, which comprises a primary winding and at least one secondary winding around a common core. The primary winding is divided into single flat coils which are provided on the core in a mutually offset way along the direction of the axis of the primary winding. To adapt the leakage inductance of the primary winding, the number of turns of the individual flat coils of the primary winding are different.
However, it has been found that the leakage inductance values cannot be increased to the desired extent by means of such an implementation of the primary winding. A transformer for an inverter (i.e. a switched-mode power supply) is known from JP-A 08-181023, particularly from its English-language abstract. In this transformer, the positions of the primary winding and the secondary winding are separated so as to vary the leakage inductance and the capacitance of the windings, by mean of which the power factor is improved and the energy losses are reduced.
Also in this arrangement, the values for the leakage inductance are limited and dimensioning cases occur for which the achievable values of the leakage inductances are not sufficient.